Heat pump systems may lose refrigerant charge by either leaks or component failure. It has been known to determine a loss of refrigerant charge by utilizing a trained technician who measures the superheat of the system using a specific procedure. There exists an acceptable range of values that are provided by the manufacturer to determine the loss of refrigerant. When the values are outside of this range, the system is low on refrigerant charge. Charge is added until the values fall in the proper range. A loss of charge is normally not checked for or analyzed until the system has a noticeable performance problem that causes consumer dissatisfaction. It has been desirable to determine the status of refrigerant charge in systems before there is this noticeable problem.
U.S. Pat. No. 5,044,168 issued Sept. 3, 1991 in the name of Wycoff discloses a method of low refrigerant charge detection. Separate pressure transducers are installed to measure the suction and discharge pressures of the compressor in the refrigeration system. When the compressor is shut off, the pressure difference between the discharge and suction pressures is measured by a microprocessor-based controller. A low refrigerant alarm or indicator is activated if the pressure difference is below a predetermined value programmed in the controller. The difference calculation could be invalid in low refrigerant load conditions when the suction and discharge pressure differences are small already.
U.S. Pat. No. 4,677,830 issued Jul. 7, 1987 in the name of Sumikawa et al. discloses an air conditioning system for vehicles to detect low refrigerant. The temperature and pressure of refrigerant at the outlet of the evaporator are detected by a temperature sensor and pressure sensor. The pressure of refrigerant detected by the pressure sensor is converted by conversion means into a corresponding saturation temperature of the refrigerant. Determining means determines whether or not the refrigerant quantity is insufficient by comparing the difference between the refrigerant temperature and the corresponding saturation temperature of the refrigerant with a predetermined reference value. Indicator means indicates abnormality when the refrigerant quantity is determined to be insufficient. If this system was applied to a heat pump, two pressure sensors and two temperature sensors would be required to handle the evaporator discharges in both heating and cooling inside.
U.S. Pat. No. 5,009,076 issued Apr. 23, 1991 in the name of Winslow discloses a refrigerant loss monitor. The monitor monitors a number of variables within the refrigerant's circuit, including environmental and refrigerant conditions, and arrives at expected refrigerant conditions utilizing the sensed environmental conditions. A pressure sensor in the compressor discharge line and a pressure sensor at either the evaporator outlet or compression suction line are measured. A computer based system compares these values to monitor refrigerant loss.